1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording material, and more particularly to an image heating apparatus adapted for use as a heat fixation device to be employed in a copying machine or a printer.
2. Related Background Art
As a fixing apparatus to be mounted in an image forming apparatus such as a copying apparatus, there has been employed a heat roller system as shown in FIG. 4. Such system has a basic configuration including a heating roller 102 for example of a metal, provided therein with a heater 101, and an elastic pressure roller 103 maintained in pressed contact therewith, and introducing and passing a recording medium, constituting a member to be heated, in a nip portion of such paired rollers for conveying therethrough, thereby fixing a toner image under heating and a pressure.
However, the fixing apparatus of such heat roller system has required a very long time for elevating the roller surface to a fixing temperature because of a large heat capacity of the roller. Therefore, in order to achieve a prompt image outputting operation, it is necessary to control the roller surface at a certain temperature even when the apparatus is not in use.
For this reason, there has been proposed a heating apparatus of a fixing film heating type, in which a film heated by a heater is utilized for fixing a developer to a recording medium. The fixing apparatus of such fixing film heating type is constituted, as shown in FIG. 5, usually of a fixing film 114 a thin heat-resistant resin (for example polyimide), a heater 113 fixedly positioned at a side of the fixing film 114, a heater holder 112 for holding the heater 113 in contact with the fixing film 114, a reinforcing stay 111 for reinforcing the heater holder 112, and a pressure roller 115 for maintaining a recording medium, constituting a member to be heated, in contact with the heater 113 across the fixing film 114.
In case it is used as a fixing apparatus, a recording medium bearing a toner image is introduced in and passed through a contact nip portion formed by a contact of the heater 113 and the pressure roller 115 across the fixing film 114, whereby an image bearing surface of the recording medium is heated by the heater 113 across the fixing film 114 to provide the unfixed image with thermal energy thereby softening and fusing the toner and fixing the image by heat.
In the fixing apparatus of such fixing film heating type, a heater of a low heat capacity can be employed because the fixing film 114 has a low heat capacity. For this reason, it is possible to reduce an electric power consumption and to shorten a waiting time to a fixable state, in comparison with apparatus of a heat roller type or a belt heating type.
In case of using the fixing film 114, it is in a slack state in a downstream side of the nip, so that it tends to follow a conveying direction of the recording medium in contact therewith. If the fixing film 114 remains in contact with the recording medium, a curvature at a separating portion decreases, thereby tending to cause a sticking jam to the fixing film 114.
In order to avoid such trouble, it is necessary, in the heat fixing apparatus of the film heating type, to facilitate the separation of the fixing film 114 and the recording medium at a downstream side of the pressed nip in the conveying direction of the recording medium. For this purpose, as shown in FIG. 5, a separating projection 112a may be provided on the heater holder 112 at a downstream end of the heater 113. In the heat fixing apparatus of the film heating type, the curvature at the separating portion is made larger in order to prevent the sticking jam. In such configuration, the fixing film 114 and the recording medium remain in close contact over a long range without pressure even after passing the nip where the pressure roller 115 is in contact, as shown in FIG. 5. However, there results a following drawback. In a portion immediately after a recording medium S is discharged from the nip of the fixing film 114 and the pressure roller 115, as shown in FIGS. 6 and 7, the recording medium S shows a thermal expansion simultaneous with the release of the recording medium S from the constriction in the nip, and an undulation Sa in the longitudinal direction appears in the recording medium S because of a difference in the expansion rate between a portion constricted in the nip and a released portion after the nip. In such undulated state, a convex portion of the recording medium S contact longer, than a concave portion, with the fixing film 114. As a result, a convex portion of the recording medium S tends to receive an excessive heat in comparison with a concave portion, as shown in FIG. 7. Such undulation Sa is more noticeable in a recording medium S of a resinous film such as an OHP sheet or a glossy film, but also appears in plain paper or glossy paper.
In case the fixing film 114 is constituted for example of a polyimide film with a very small heat capacity (for example a thickness of 50 μm and a heat capacity per unit area of 0.01 J/cm2·K), a difference in the amounts of heat received by such convex portion and concave portion is small and does not exert a significant influence on the image.
However, in case of employing a fixing sleeve constituted of an elastic layer, a releasing layer and a metal film, having a certain heat capacity (for example a heat capacity per unit area of 0.1 J/cm2·K), a convex portion in an undulation Sa generated in the recording medium S receives excessive heat in comparison with a concave portion. Such excessive heat deteriorates a surface smoothness of the recording medium S in a convex portion thereof, thereby deteriorating a transparency along the convex portion of the undulation Sa as shown in FIG. 8 in case the recording medium S is an OHP sheet, or generating an unevenness in the glossiness in case the recording medium S is a glossy film. Also in an ordinary recording paper, a thermal offset is generated in the convex portion.